William also mentions other spirits associated with the night, with death, and with women, such as lamiae and striges believed to kill infants, or the Ephialtes, a form of demon believed to injure sleepers.

Equally important, in treating these phenomena William reserves space for divine providence, arguing that God sometimes uses demons as agents of vengeance against sinners, either mimicking disease in the case of Ephialtes or destroying infants in the case of the striges.

Much like modern grey aliens, the Ephialtes was a variety of demon believed to sit on the chest, causing sleep, paralysis, or death. And just as it was the case in the thirteenth century, attacks of grey aliens is nowadays attributed to medical causes rather than demons or aliens.